PROCESS FOR PREPARING EMULSION POLYMERS HAVING A HOLLOW STRUCTURE

Abstract: The present invention relates to a process for preparing emulsion polymers having a hollow structure. More specifically, the present invention relates to a process for preparing emulsion polymers which comprises forming a core which is an alkali-swellable resin, carrying out a subsequential emulsion polymerization by means of a continuous variation in composition and a kinetic steric barrier to form a hard polymer shell outside the core, swelling the core with an alkali and then drying to remove water contained in the alkali-swellable core, thereby forming a closed internal pore in the center of polymer.

Description: PROCESS FOR PREPARING EMULSION POLYMERS HAVING A HOLLOW STRUCTURE TECHNICAL FIELD The present invention relates to a process for prepar¬ ing emulsion polymers having a hollow structure. More specifically, the present invention relates to a process for preparing emulsion polymers which comprises forming an internal layer with an alkali-sweilable resin, carrying out a subsequential emulsion polymerization by means of a continuous variation in composition and a kinetic steric barrier to form a hard polymer layer outside the internal layer, swelling the internal layer with an alkali and then drying to remove water contained in the alkali-sweilable resin, thereby forming a closed internal pore in the center of polymer particle. BACKGROUND ART Emulsion polymers having a hollow structure have been widely used as a plastic pigment in the field of water- soluble paints, paper coating agents, information record¬ ing papers, synthethic resins and the like. In the prior art, numerous methods for preparing plas tic pigments having a hollow structure have been proposed as follows : 1) Method for preparing a hollow onocellular particle comprising polymerizing a mixture of monomers contain¬ ing a blowing agent to prepare a plastic bead which is then blown under heating (see U.S. Patent Specifica- tion No . 3 , 615 , 972 ) ; 2) Method for preparing a polyester bead with a hollow having a diameter of 1 to 25 micron which comprises mechanically stirring a solution of polyester dis¬ solved in styrene to prepare an emulsion of water/oil/water which is then polymerized (see U.S. Patent Specification No. 3,891,577); 3) Method for preparing a hollow particle comprising adding a volatile material to a polymer and then gasifying the volatile material to swell the polymer (see Japanese Patent No. 252635/1985) ; 4) Method for including vesicles into a dissolved polymer by dissolving a polymer and then blowing a gas such as air, etc. , therein; and 5) Method for preparing a hollow particle which comprises swelling a polymer particle having a glass transition temperature of -10°C or more with hot air in the spray dryer and then immediately cooling the swollen polymer to normal temperature (see Japanese Laid-open Patent Publication No. (Hei) 4-145131). Although the polymer beads prepared according to the prior methods as mentioned above have a self-hiding property and therefore can serve as a pigment, those prior methods have some disadvantages in that the particle size cannot be uniformly controlled, and those methods cannot properly play a role of a dispersing agent of titanium oxide, poorly contributes to a critical surface porosity and tends to precipitate during the storage, and further the pore size and shell thickness are difficult to con- trol. In addition, Japanese Patent Publication No. 44282/88 discloses a method for preparing a hollow particle by phase separation and polymerization shrinkage. However, this method has also disadvantages that the hollow portion is not smooth and the method cannot be practically used in industrial scale because of the high material cost result¬ ing from the massive use of cross-linking agent. In order to improve the disadvantages in such prior methods for preparing plastic pigments having hollow particles, a method for producing hollow particles by preparing a core-shell polymer having an alkali-sweilable resin inside the particles via a multi-step continuous polymerization and then subjecting the polymer to alkali swelling and drying has been developed. This multi-step continuous polymerization is influenced by the composition and glass transition temperature of monomers used in each polymerization step, the kind and concentration of an emulsifying agent, the kind and concentration of a poly¬ merization initiator, reaction temperature, etc. Fur- ther, the method using alkali-swelling procedure tends to produce a reverse core-shell polymer or a confetti-like abnormal structure due to the hydrophilicity of polymers present inside the particles, which is far greater than that of polymers present on the outer shell of particles. Accordingly, numerous methods have been proposed for inhibiting the formation of such abnormal particle struc¬ ture. For example, U.S. Patent Specification No. 4,427,836 discloses a method which comprises polymerizing carboxylic acid-containing hydrophilic polymers of which the volume can be swollen by two times or more with an aqueous solu¬ tion of volatile base at the first emulsion polymerization step and then forming a sheath of a polymeric material through which the aqueous base solution can be permeated, on the surface of a core by means of thermal or redox initiation emulsion polymerization at the secod emulsion polymerization step. However, this method has the disad¬ vantages in that additional new particles are greately generated, the stability of latex is poor, and the shell- forming monomers should be used in an amount of 10 times or more the core-forming monomers for providing a uniform core-shell structure and therefore the shell becomes thick. In addition, Korean Patent Publication No. 87-1808 discloses a method wherein a core is formed from the monomer mixture containing monomer having a carboxylic acid group and then at a shell-forming step, in order to effectively inhibit the formation of abnormal particles a copolymer surfactant is used as an emulsifier and the feeding rate of monomers, the kind of polymerization initiator and the reaction temperature are specifically selected. According to this method, the formation of new particles in each polymerization step can be inhibited. However, this method is not greately useful for improve- ment in the hiding property since the copolymer surfact¬ ant, which is a strong electrolyte, present on the surface of core causes an increase in the heterogeneity between the core polymer and the shell polymer which makes the adhesion of hydrophobic shell and hydrophilic core poor. Meanwhile, Korean Patent Publication No. 93-830 dis¬ closes a method for preparing emulsion polymer by forming a hydrophilic core with a seed formed from an acid monomer and a vinyl monomer and providing a hard sheath layer with a hydrophobic monomer outside the core, wherein the emul¬ sion polymerization is carried out by adding a group transfer polymerization initiator or an oil-soluble initi¬ ator when the seed and the core/sheath layer are produced by emulsion polymerization in the aqueous system. This method can reduce the agglomeration of latex particles but has some disadvantages that the use of oil-soluble initia¬ tor results in production of new suspension particles and the difference in hydrophilicity of core and sheath poly¬ mers makes the encapsulation efficiency poor. DISCLOSURE OF INVENTION Thus, the present inventors have extensively studied to develop a method which can prevent lowering of the encapsulation efficiency due to the great difference in hydrophilicity between a core polymer and a shell polymer and can produce monodisperse hollow particles of emulsion polymer having a uniform hollow size and a uniform shell thickness. As a result, we have identified that the encapsulation efficiency can be maximized by adding a cross-linking agent to the core and shell layers to mini¬ mize the mutual interdiffusion of polymer chains, while the hydrophilicity is continuously altered from the inner layer to the outer layer of the shell, and therefore monodisperse hollow particle of emulsion polymer consist¬ ing of a solid hydrophobic shell and having uniform hollow size and uniform shell thickness can be produced. Accordingly, it is an object of the present invention to provide an improved process for preparing emulsion polymers having hollow structure. It is another object of the present invention to provide a process for preparing emulsion polymers which comprises forming a core polymer with an alkali-sweilable resin, carrying out subsequential emulsion polymerization by means . of continuous variation in composition and a kinetic steric barrier to form hard polymer shell outside the core, swelling the core with an alkali and then drying to remove water contained in the alkali-swollen core, thereby forming a closed internal pore in the center of polymer particle. It is still another object of the present invention to provide a process " for preparing the hollow particles of emulsion polymer which comprises a seed-forming step for polymerizing the monomer mixture of an acid monomer having a carboxylic acid group and a non-ionic hydrophilic monomer in the presence of a polymerization initiator; a core-forming step for preparing a core, which can be swollen with an alkali, by polymerizing the monomer mix¬ ture of an acid monomer having a carboxylic acid group, a non-ionic hydrophilic monomer and a cross-linking agent to the seed latex; a step for forming the hydrophobic shell on the hydrophilic core; and an alkali-swelling step. Further, it is another object of the present invention to provide a process for preparing hollow particles of the emulsion polymer having a single hollow in the center and a uniform thickness of the shell, wherein the process comprises a seed-forming step polymerizing the mixed monomers of an acid monomer (i) having a carboxylic acid group and a non-ionic hydrophilic monomer (ii) in the presence of a polymerization initiator; a core-forming step for preparing a core, which can be swollen with an alkali, by polymerizing the monomer mixture of an acid monomer (i) having a carboxylic acid group, a non-ionic hydrophilic monomer (ii) and a cross-linking monomer (iii) to the seed latex; a step for forming the hydrophobic shell on the hydrophilic core; and an alkali-swelling step, characterized in that in the core/shell-forming steps an emulsion (I) consisting of up to 10 wt% of a single or mixed acid monomer (i) having a carboxylic acid group, 85 to 100 wt% of a single or mixed non-ionic hydro¬ philic monomer (ii) and up to 5.0 wt% of a cross-linking monomer (iii) and an emulsion (II) consisting of 95 to 100 wt% of a single or mixed hydrophobic monomers (iv) and up to 5 wt% of a cross-linking agent (iii) are used in the ratio of 1/1-1/10 [emulsion (I) /emulsion (II)], the total amount of the emulsion (I) and the emulsion (II) is two times or more the weight of the core polymer, and the polymerization is carried out by continuously adding the emulsion (II) dropwise to the emulsion (I) and, at the same time, adding the emulsion (I) together with a water- soluble initiator to the core latex at the same rate as the addition of the emulsion (II) to the emulsion (I) . BEST MODE FOR CARRYING OUT THE INVENTION The process for preparing the hollow particles of emulsion polymers according to the present invention comprises a seed-forming step for polymerizing the monomer mixture of an acid monomer having a carboxylic acid group and a non-ionic hydrophilic monomer in the presence of a polymerization initiator; a core-forming step for prepar¬ ing a core, which can be swollen with an alkali, by adding the monomer mixture of an acid monomer having a carboxylic acid group, a non-ionic hydrophilic monomer and a cross- linking agent to the seed latex; a step for forming the hydrophobic shell on the hydrophilic core; and an alkali- swelling step. Although the seed-forming step and the core-forming step can be practiced according to conventional emulsion polymerization methods, the formation of the hydrophobic shell on the hydrophilic core polymer is substantially difficult in a technical view. That is, since the core polymer and the shell polymer have no miscibility due to great difference in their hydrophilicity , in general , the shell polymer cannot completely encapsulate the core polymer, and therefore, the abnormal core-shell structure , in which the core polymer is partially encapsulated by the shell polymer, or discrete particles of the core polymer and the shell polymer, in which the core polymer is not encapsulated by the shell polymer, may be produced. Accordingly, in the present invention, the shell is formed by providing an emulsion (I) consisting of an acid monomer, a non-ionic hydrophilic monomer and a cross- linking agent, and an emulsion (II) consisting of a hydro¬ phobic monomer and a cross-linking monomer and then, when the shell-forming monomers are added to the core latex, slowly adding the emulsion (II) dropwise to the emulsion (I) and, at the same time, adding the emulsion (I) partly containing emulsion (II) to the core latex at the same rate as the addition of the emulsion (II) to the emulsion (I) . According to this procedure, the production of particles having abnormal structure due to the difference in hydrophilicity can be prevented, the mutual interdiffu¬ sion of polymer chains due to cross-linking can be mini¬ mized and further the viscosity at the polymerization loci can be optimally controlled to maximize the encapsulation efficiency. Then, the hollow particles of emulsion polymer accord¬ ing to the present invention can be prepared by swelling the final latex with alkaline solution. Since the re- suiting hollow particles are completely encapsulated by the hard hydrophobic shell, the destruction of hollow during the drying the latex can be prevented and therefore the final latex can be used in the area of paints and paper coating industries. Hereinafter, the present invention will be more spe¬ cifically illustrated. The preparation of the seed latex is the pre-step for praparing the core latex. In this step, the monomer mixture of the acid monomer (i) and the non-ionic hydro¬ philic monomer (ii) is polymerized in the presence of an emulsifier and a water-soluble polymerization initiator. As the acid monomer (i) for this purpose, any one selected from the group consisting of unεaturated carboxylic acids having a reactive double bond such as acrylic acid, metha- crylic acid, crotonic acid, fumaric acid and itaconic acid can be used alone or in combination with one or more other monomers selected therefrom . Such acid monomer (i) is used in an amount of up to 10 wt% based on the weight of the monomer mixture. The non-ionic hydrophilic monomers (ii) which can be used in the present invention can include methyl acrylate, ethyl acrylate, butyl (meth)acrylate, methyl methacrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acry- ate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, acry- lonitrile, vinyl acetate, vinyl pyridine, acrylamide, methacrylamide, N-methylol (meth)acrylamide, and the like. The non-ionic hydrophilic monomers should have a water- solubility of at least of 0.5%, more preferably at least of 1%. In the present invention, any non-ionic hydor- philic monomer selected from the above mentioned monomers can be used alone or in combination with one or more other monomer selected therefrom. A polymerization initiator to be used for the seed latex and carrying out the subsequent reaction steps can be suitably and optionally selected from conventional initiators for thermal and redox reaction. Specific examples of the polymerization initiator which can be used in the present invention includes ammonium persulfate, potassium persulfate and sodium persulfate. Such poly¬ merization initiator can also be used in the combination with a reducing agent such as sodium bisulfite or sodium formaldehyde sulfoxylate. The reaction temperature is maintained preferably at 70 to 90°C when ammonium persul¬ fate, potassium persulfate or sodium persulfate is used alone as the initiator. Alternatively, when the initiator is used in the combination with a reducing agent selected from sodium bisulfite and sodium formaldehyde sulfoxy- late, the reaction temperature can be controlled prefera- bly in the range of 30 to 70°C. As the emulsifier an anionic, non-ionic or cationic emulsifier which is conventionally used in the emulsion polymerization can be used either alone or as the mixture thereof. Specific preferred example of the emulsifier which can be used in the present invention include rhodi- nates such as potassium rhodinate, sodium rhodinate, etc. ; aliphatic sodium or potassium salts such as potassium oleate, potassium laurate, sodium laurate, sodium stear- ate, potassium stearate, etc. , and sulfuric acid ester salts of aliphatic alcohols such as sodium lauryl sulfate; an anionic emulsifier, for example, alkylaryl sulfonates such as sodium dodecylbenzenesulfonate, sodium dialkylsul- fosuccinate, formalin condensate of naphthalene sulfonic acid, and the like; a non-ionic emulsifier, for example, alkyl esters, alkyl ethers, alkylphenyl ethers, etc., of polyethyleneglycol, and the like. Among those emulsifi- ers, sodium lauryl sulfate, sodium dodecylbenzenesulfo- nate, sodium dioctylsulfosuccinate, formalin condensate of naphthalene sulfonic acid, alkylphenyl ethers of polyet¬ hyleneglycol, etc. can be more preferably used. As a cationic emulsifier, quaternary ammonium salts can be preferably used. However, when the aqueous dispersion is cationic, the cationic emulsifier can be used alone or together with a non-ionic emulsifier. The emulsifier is preferably used in an amount of 0.1 to 10 wt% on the basis of the weight of monomers. In the core-forming step the monomer mixture consist- ing of the acid monomer, the hydrophilic monomer and the cross-linking agent is used. For this purpose, the acid monomer which can be used is any one selected from the acid monomer (i) as mentioned above, either alone or in combination with one or more other monomers selected therefrom, and is used preferably in an amount of 5 to 40 wt% on the basis of the weight of the monomer mixture. As the hydrophilic monomer, one selected from the hydro¬ philic monomer (ii) mentioned hereinbefore can be used either alone or in combination with one or more other monomer selected therefrom, preferably in an amount of 60 to 95 wt% on the basis of the weight of the monomer mix- ture. In this step, the cross-linking agent (iii) which can be used is selected from divinyl benzene, ethylengly- col di(meth)acrylate, trimethylol propane trimethacrylate, hexamethyleneglycol diacrylate, allyl methacrylate and the like. The cross-linking agent can be preferably used in an amount of 0.1 to 5.0 wt% on the basis of the weight of the monomer mixture. The monomer mixture consisting of such acid monomer, hydrophilic monomer and cross-linking agent is slowly added to the seed latex together with the emulsifier and the water-soluble initiator to obtain the monodisperse core latex which can be swollen with an alkaline solution afterwards. Preferably, the monomer composition which can be used in the emulsion (I) for the shell-forming step consists of up to 10 wt% of the acid monomer which is one, or the mixture of two ro more, selected from the acid monomer (i) mentioned hereinbefore; 85 to 100 wt% of the hydrophilic monomer which is one, or the mixture of two or more, selected from the hydrophilic monomer (ii) mentioned hereinbefore; and up to 5.0 wt% of the cross-linking agent selected from the cross-linking agent (iii) mentioned hereinbefore. The monomer mixture which is used in the emulsion (II) consists of the hydrophobic monomer and the cross-linking agent. As hydrophobic monomers (iv) for this purpose, one, or the mixture of two or more, selected from aromatic vinyl monomers such as styrene, methyl styrene, ethyl styrene, vinyl toluene, chlorostyrene, vinyl naphthalene, etc., can be used in an amount of 95 to 100 wt% on the basis of the weight of the monomer mixture. As the cross-linking agent for this purpose, any of the cross- linking agent (iii) is used preferably in an amount of up to 5.0 wt% on the basis of the weight of the monomer mixture. The ratio of the emulsion (I) and the emulsion (II) is preferably in the range of 1/1 to 1/10 [emulsion (I)/ emulsion (II) ] . The total mixture of monomers for form- ing the shell polymer is used preferably in an amount of two times or more an amount of the core polymer on the basis of weight. The emulsion (I) and the emulsion (II) thus prepared are added to the core latex as described below. Specifi¬ cally, the emulsion (II) is slowly added to the emulsion (I) and, at the same time, the emulsion (I) partly con¬ taining emulsion (II) is added to the core latex together with the water-soluble initiator, at the same rate as the addition of the emulsion (II) to the emulsion (I) , to obtain the monodisperse emulsion polymer having a perfect core/shell structure. The resulting latex having the core/shell structure is adjusted to pH 6 to 12 with aqueous ammonium or a volatile base such as triethylamine, diethanolamine, triethanola- mine, etc, or an organic base, swollen for 30 minutes or more at temperature in the range of 60 to 100°C and then dried to obtain the hollow particles having an outer diameter of 0.1 to 5 micron, an inner diameter of 0.05 to 4 micron and the inner diameter/outer diameter ratio of 0.1 to 0.9 and having a uniform shell thickness. The particle and hollow size of the hollow latex particles obtained according to the present invention are observed by means of a transmission electron microscope (TEM) . The present invention is more specifically explained by the following examples. However, it should be under¬ stood that the present invention is not limited to those examples in any manner. EXAMPLE 1 (1) Production of the seed latex To a 2L four-necked flask equipped with a stirrer, a thermometer, a refluxing condenser and a nitrogen inlet 300g of deionized water was added and then the flask was purged with nitrogen gas while maintaining the temperature of 75°C. The emulsion consisting of 15g of deionized water, 15g of methyl methacrylate, 0.3g of methacrylic acid and 0.3g of an aqueous solution of sodium dodecylben- zene sulfonate (solid content : 12 wt%) , which was pre¬ pared in advance, was introduced into the flask and then an aqueous solution of 0.12g of potassium persulfate dissolved in 15g of deionized water was added thereto. After the addition is completed, the reaction mixture was polymerized for 2 hours to obtain the monodisperse emul¬ sion polymer having an average particle diameter of 0.08 micron. (2) Production of the core latex To the flask containing the seed latex was added 600g of deionized water. Thereafter, the emulsion of 168g of methyl methacrylate, 72g of methacrylic acid, 1.2g of ethyleneglycol dimethacrylate and 7.5g of sodium dodecyl- benzene sulfonate (solid content : 12 wt%) in 144g of deionized water an the aqueous solution of 0.96g of potassium persulfate dissolved in 120g of deionized water were slowly added dropwise thereto at the same rate over 4 hours at 75°C. The reaction mixture was polymerized for 2 hours to obtain the monodisperse emulsion polymer having an average particle diameter of 0.20 micron. (3) Production of the core/shell latex 171g of the core latex (solid content : 17.5 wt%) , as prepared above, and 375g of deionized water were added to a 2L four-necked flask and then heated to 75°C while purging the flask with nitrogen. Separately, two kinds of the emulsion were prepared. One is the emulsion (I) prepared from 90g of methyl metha¬ crylate, 2.7g of sodium dodecylbenzene sulfonate (solid content : 12 wt%) and 54g of deionized water and the other is the emulsion (II) prepared from 180g of styrene, 1.5g of divinyl benzene, 5.4g of sodium dodecylbenzene sulfo¬ nate (solid content : 12 wt%) and 108g of deionized water. The prepared emulsion (II) was added to, and mixed with, the emulsion (I) at the constant rate and, at the same time, the emulsion (I) of which the composition is continuously varied according to the addition of the emulsion (II) was continuously added dropwise to the flask at the same rate as the addition of the emulsion (II) to the emulsion (I) , over about 5 hours. Simultaneously, the aqueous solution of 1.08g of potassium persulfate dissolved in 135g of deionized water in another flask was added to the flask over the same period as the addition of the emulsion (I) . Thereafter, the reaction mixture was allowed to polymerize and then aged for 2 hours to obtain the core/shell latex. The obtained core/shell latex comprises the monodisperse emulsion polymer of which the solid content is 25.6 wt% and the average particle diame¬ ter is 0.45 micron. (4) Alkali swelling The core/shell latex was continuously heated to 90°C and then adjusted to pH 10.8 by adding aqueous ammonium (solid content : 28 wt%) and swollen for 2 hours. The obtained latex was identified as being the monodisperse, spherical hollow particle having a hollow size of 0.3 micron and an average latex particle diameter of 0.5 micron. EXAMPLE 2-7 The hollow particles were prepared by a multi-step emulsion polymerization according to the same procedures as Example 1 except that the emulsion (I) and the emulsion (II) having the compositions as described in Table 1 are used in perparation of the core/shell particle. The obtained results are described in Table 1. EXAMPLE 8 The hollow particle was prepared by a multi-step emulsion polymerization according to the same procedures as Example 1 except that the emulsion (I) and the emulsion ( II ) having the compositions as described in Table 1 are used in perparation of the core/ shell particle and the aqueous initiator solution comprises 0 . 9g of potassium persulfate and 112g of deionized water . The obtained results are described in Table 1. EXAMPLE 9 The hollow particle was prepared by a multi-step emulsion polymerization according to the same procedures as Example 1 except that the emulsion (I) and the emulsion (II) having the compositions as described in Table 1 are used in perparation of the core/shell particle and the aqueous initiator solution comprises 1.8g of potassium persulfate and 225g of deionized water. The obtained results are described in Table 1. EXAMPLE 10 The hollow particle was prepared by a multi-step emulsion polymerization according to the same procedures as Example 1 except that the emulsion (I) and the emulsion (II) having the compositions as described in Table 1 are used in perparation of the core/shell particle and the aqueous initiator solution comprises 0.72g of potassium persulfate and 90g of deionized water. The obtained results are described in Table 1. Table 1. Composition of the emulsions used in preparing the core/shell latex and the characteristics of the hollow particle prepared therefrom - y -j Note : 1) outer diameter = particle size 2) inner diameter = hollow size 3) MMA = methyl methacrylate 4) MAA •= methacrylic acid 5) EGDMA = ethyleneglycole dimethacrylate 6) SDBS a.s. - 12 wt% aqueous solution of sodium dodecylbenzene sulfonate 7) ST = styrene 8) DVB = divinyl benzene

Claims: WHAT IS CLAIMED IS : 1. A process for preparing hollow particles of the emul- sion polymer having a single hollow in the center and a uniform thickness of hollow and shell, wherein the process comprises a seed-forming step for polymerizing the mixed monomers of an acid monomer (i) having a carboxylic acid group and a non-ionic hydrophilic monomer (ii) in the presence of a polymerization initiator; a core-forming step for preparing a core, which can be swollen with an alkali, by polymerizing the monomer mixture of an acid monomer (i) having a carboxylic acid group, a non-ionic hydrophilic monomer (ii) and a cross-linking agent (iii) to the seed latex; a step for forming the hydrophobic shell on the hydrophilic core; and an alkali-swelling step, characterized in that in the core/shell-forming steps an emulsion (I) consisting of up to 10 wt% of a single or mixed acid monomer (i) having a carboxylic acid group, 85 to 100 wt% of a single or mixed non-ionic hydrophilic monomer (ii) and up to 5.0 wt% of a cross- linking monomer (iii) and an emulsion (II) consisting of 95 to 100 wt% of a single or mixed hydrophobic monomer (iv) and up to 5 wt% of a cross-linking agent (iii) are used in the ratio of 1/1-1/10 [emulsion (I) /emulsion (II)], the total amount of the emulsion (I) and the emulsion (II) is two times or more the weight of the core polymer, and the polymerization is carried out by continuously adding the emulsion (II) dropwise to the emulsion (I) and, at the same time, adding the emulsion (I) partly containing emulsion (II) together with a water-soluble initiator to the core latex at the same rate as the addition of the emulsion (II) to the emulsion (I) . 2. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the hollow particle as prepared has an outer diameter of particle of 0.1 to 5 micron, a diameter of hollow (inner diameter) of 0.05 to 4 micron, and the inner diameter/outer diameter ratio of 0.1 to 0.9. 3. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the seed-forming step is carried out by poly¬ merizing the monomer mixture consisting of the acid monomer (i) having a reactive double bond and the non- ionic hydrophilic monomer (ii) , wherein the acid monomer (i) is present in an amount of up to 10 wt%, in the presence of an emulsifier and a water-soluble polymerization initiator. 4. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the step for preparing the core emulsion polymer is carried out by slowly adding dropwise the monomer mixture consisting of the acid monomer (i) , the hydrophilic monomer (ii) and the cross-linking agent (iii) , wherein the acid monomer (i) is a single component or in the form of a mixture and is present in an amount of 5 to 40 wt% on the basis of the weight of the monomer mixture, the hydrophilic monomer is present in an amount of 60 to 95 wt% on the basis of the weight of the monomer mixture and the cross-link- ing agent is present in an amount of 0.1 to 5.0 wt% on the basis of the weight of the monomer mixture, to¬ gether with an emulsifier and a water-soluble polymer¬ ization initiator to the seed latex to obtain the monodisperse core latex which can be swollen by an alkali afterwards. 5. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the alkali-swelling step is carried out by adjusting the pH value of the latex having a core/shell structure with a volatile base or an organ- ic base in the range of 6 to 12 and then swelling the latex for 30 minutes or more at temperature of 60 to 100°C. 6. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the acid monomer (i) is selected from acrylic acid, methacrylic acid, crotonic acid, fumaric acid and itaconic acid. 7. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the non-ionic hydrophilic monomer (ii) has the water solubility of 0.5 wt% or more. 8. The process for preparing hollow particles of the emulsion polymer according to claim 1 or 7, character¬ ized in that the non-ionic hydrophilic monomer (ii) is one or the mixture of two or more selected from the group consisting of methyl acrylate, ethyl acrylate, butyl (meth)acrylate, methyl methacrylate, 2-hydrox- yethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, acrylonitrile, vinyl acetate, vinyl pyridine, acryla- mide, methacrylamide and N-methylol (meth)acrylamide. 9. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the cross-linking agent (iii) is selected from the group consisting of divinyl benzene, ethylenegly- col di(meth)acrylate, trimethylol propane trimethacry- late, hexamethyleneglycol diacrylate and allyl metha- cry late. 10. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the hydrophobic monomer (iv) is selected from the group consisting of styrene, methyl styrene, ethyl styrene, vinyl toluene, chlorostyrene and vinyl naph- thanene. ii. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the emulsifier is used in an amount of 0.1 to 10 wt% on the basis of the weight of the total mono¬ mers used in each step. 12. The process for preparing hollow particles of the emulsion polymer according to claim 1, characterized in that the water-soluble polymerization initiator is ammonium persulfate, potassium persulfate or sodium persulfate alone or in a combination with a reducing agent selected from sodium bisulfate and sodium for¬ maldehyde sulfoxylate.

Inventors: CHOI SU BEOM, JANG TAE HWAN, LEE CHAN HONG, YOO JIN NYOUNG, CHOI, SU, BEOM, JANG, TAE, HWAN, LEE, CHAN, HONG, YOO, JIN, NYOUNG
Applicants: CHOI SU BEOM, JANG TAE HWAN, LEE CHAN HONG, LUCKY LTD, YOO JIN NYOUNG, CHOI, SU, BEOM, JANG, TAE, HWAN, LEE, CHAN, HONG, LUCKY LTD., YOO, JIN, NYOUNG
Application year: 1994
Main classification: C08F285
IPC codes: C08F2, C08F6, C08F265, C08F285, C08F291, C08J9